1. Field of the Invention
This invention relates to a portable medium such as a multifunctional integrated circuit (IC) card incorporating a central processor unit (CPU), a data memory, a battery and so on, which is used for calculation and timekeeping, and adapted for use with terminals.
2. Description of the Related Art
Recently, multifunctional IC cards have been developed which incorporate a CPU, a data memory, a battery and so on. The IC card itself serves as a hand-held calculator or timepiece with the aid of a keyboard and display device mounted on the top thereof. Also, the IC card may be inserted into a terminal for data transfer.
European Patent Disclosure No. 0167044 (published Aug. 1, 1986) discloses an IC card with a timepiece for displaying timepiece data such as date and time, and a calculator for performing arithmetic operations.
Where the IC card is used alone, or used in off-line operations, data processing is performed by the use of an internal battery.
The IC card includes a battery checking circuit for checking whether the voltage of the battery is above a specified voltage. The battery checking circuit continuously generates a reference voltage by the use of a Zener diode. The voltage of the battery is compared with the reference voltage, whereby the battery voltage is checked.
However, the battery checking circuit does not produce accurate voltage as the reference voltage if the electric current to the Zener diode is less than a predetermined value.
More importantly, the battery power is dissipated due to the continous nature of the reference voltage, and battery life is shortened.